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JIM ROBBINSSCIENCE10.16.2021 08:00 AM Ground-Level Ozone Is a Creeping Threat to Biodiversity

Ground-Level Ozone Is a Creeping Threat to Biodiversity

Scientists are learning how this pollutant damages plants and trees, setting off a cascade of effects that harms everything from soil microbes to wildlife.
Sequoia Tree
Giant sequoias in Sequoia National Park, California.PHOTOGRAPH: MARJI LANG/GETTY IMAGES

THIS STORY ORIGINALLY appeared on Yale Environment 360 and is part of the Climate Desk collaboration.

Sequoia National Park’s famous groves of stout, 300-foot-tall trees sit high on the western side of the Sierra Nevada, above California’s San Joaquin Valley. They are threatened as never before: Wildfires have burned much of the forest, and now, for the first time, insects are killing sequoias.

There is also a stealthier threat to these majestic trees and the forest ecosystem of which they are a part. Ozone levels at Sequoia and the adjacent national park, King’s Canyon, are among the highest in the United States, thanks to smog that blows in from the urban areas and farming and industrial activity in the San Joaquin Valley below. Smog levels here are sometimes as high, or higher, than they are in Los Angeles.


It has long been known that ground level, or tropospheric, ozone damages trees and other plants by affecting a host of biological processes at the cellular level. Studies have shown that high ozone levels negatively impact plant growth, vitality, photosynthesis, water balance, the flowering process, and the abilities of plants to defend themselves.

More recently, researchers have turned their attention to how the detrimental effects of ozone on flora can ripple through entire ecosystems and impact biodiversity, harming insects, wildlife, and even soil.


Studies show that those knock-on effects can include making plants less nutritious; diminishing the scent trails pollinators follow to find their target; changing the timing of leaf fall, affecting the forest floor and the microbial communities that inhabit it; impacting the root systems of plants and trees and the microbes, fungi, and other organisms that live there; and even reducing harvests of staple food crops such as corn and wheat. And scientists predict that these negative effects will grow worse as the planet warms, since ground-level ozone increases as temperatures rise.

The impacts from ozone in the Sierras and elsewhere are far from fully understood, because the effects are difficult to study, and research hasn’t been well funded. It can be difficult to tease out the effects of ozone from other stressors such as drought and warmer temperatures, and many of the changes from ozone may not be detectable for years or decades.

Sequoia National Park

The view from Beetle Rock in Sequoia National Park, California. Smog, containing high levels of ozone, blows in from the San Joaquin Valley.


But research to date shows that elevated ozone levels are already taking a toll on biodiversity in myriad ways.


Ozone is the most damaging pollutant in the world,” said Evgenios Agathokleous, a professor of environmental resources at the Institute of Ecology at Nanjing University of Information Science & Technology in China and one of the top researchers in the field. “It induces the most widespread damage to plants, and it’s a very serious threat to biodiversity.” In some parts of Asia, he said, ozone levels are 10 times the critical thresholds.

Ozone—an invisible, odorless gas in the upper atmosphere—is essential for life because it shields the earth from the sun’s harmful ultraviolet rays. The 1987 Montreal Protocol phasing out industrial chemicals that were destroying the ozone layer is considered one of the most successful international environmental actions ever taken. At ground level, though, ozone is toxic to life. “Good up high, bad nearby” is the phrase some scientists use. It is also a greenhouse gas—the third worst after carbon dioxide and methane.

Ground-level ozone is created mostly by humans, formed by pollutants—such as nitrous oxide, methane, and volatile organic compounds—that are emitted by cars, trucks, refineries, power plants, and oil and gas development. It’s chemically similar to chlorine, but not as toxic.

When these airborne pollutants meet sunlight and warm temperatures, they go through a chemical reaction and become ozone. In the US, ozone has decreased substantially in recent decades thanks to regulations governing the emission of nitrous oxide and other precursor pollutants. Yet there are places where ozone remains at high levels, including much of the West. In addition to Sequoia National Park, ozone levels remain very high in other parks, including Joshua Tree and Rocky Mountain National Parks. Denver, Salt Lake City, and Albuquerque, among other urban areas in the West, often reach health-threatening levels of ozone.

Studies show ozone levels are increasing steadily across the entire Asia-Pacific region, Africa, and Europe. The source of most of it is China, India, and southeast Asia. And ozone can travel vast distances—rising up from northern India to envelop the Himalayas or crossing the Pacific from Asia to become a factor in ozone levels on the West Coast, including the ozone affecting sequoias. Ozone has also been cited in damage to plants in the Arctic.

In addition, warmer weather increases ground-level ozone, a problem that is predicted to worsen. “If you are under polluted conditions as the climate warms, you get more ozone,” said Daniel Jacobs, a professor of atmospheric chemistry at Harvard University who studies ozone, air pollution, and methane. “Three reasons: there is more stagnation and accumulation of polluted air, the reactions that cause ozone happen faster as it gets warmer, and nitrous oxide (a precursor) has a longer lifetime in warmer temperatures.” Worsening air pollution from climate change is sometimes referred to as the “climate penalty.”

Meanwhile, methane levels in the atmosphere have been increasing rapidly since 2006 and last year hit record highs. In addition to being a potent greenhouse gas, methane is also an ozone precursor.


It’s well established that chronic exposure to high ozone levels is a serious threat to human health, exacerbating heart and lung problems such as asthma and emphysema, and causing decreased birth weights. One study found that more than 1 million premature deaths are caused globally each year by high levels of ozone.


Research also shows that crops and forests are damaged or killed by ozone, either directly or indirectly, as ozone makes them more susceptible to insects, disease, and drought. Ozone does more damage to plants than all other air pollutants combined, according to the US Department of Agriculture. The gas is predicted to cause a substantial decline in global food production. One recent study predicted that by 2050, wheat yields would decline by 13 percent, soybeans by 28 percent, and corn by 43 percent because of rising temperatures and ozone.

While it’s clear that ozone can take a toll on all living organisms, research has not, until recently, looked at its effects on biodiversity. Scientists believe, however, that the impacts are substantial. This month the International Union of Forest Research Organizations, a global network of scientists, is holding a conference titled Air Pollution Threats to Plant Ecosystems. Ozone is at the top of the list.

In a paper published last year, 20 researchers in Europe and Asia, including Agathakleous, modeled what could happen to ecosystems in coming decades as a result of ozone pollution. They concluded that ozone will affect “the composition and diversity of plant communities by affecting key physiological traits” and can cause a cascade of changes that diminish biodiversity. In their paper, the researchers urged officials to take ozone into account in efforts to protect and restore biodiversity and said its effects should be included in assessments of atmospheric pollution and climate change.

Research is showing that ozone affects plants in a wide variety of ways.

“It paralyzes the plants’ stomata,” said Howard Neufeld, a plant ecologist at Appalachian State University, “and so they release more water than they take in.” Stomata are the microscopic openings on the surface of leaves where trees exchange gases with the atmosphere. Ozone damages them and interferes with a variety of processes, including photosynthesis.

Ozone also damages leaves and accelerates their aging. “As leaves are injured, photosynthesis goes down; a plant makes less sugars, and it has fewer resources,” says Neufeld. “It also affects the movement of sugars to roots, which reduces root growth, making them more susceptible to drought and nutrient deficiencies and disease.”

Ozone damage can also alter the timing of leaf fall and shrink leaf size, reducing the amount of litter and affecting the microbial communities that thrive in decomposing leaves. Microbes in the litter and soil are critical to taking up nutrients, helping trees resist disease and use water efficiently.

Ozone’s impacts on soil also affect the rhizosphere—the root system and its associated microbes, fungi, and other organisms. “When the plants respond to ozone, they consume energy,” said Agathokleous. “When they use so much energy, there is less to provide to organisms in the soil, and the chemical composition can be affected.” Less nutritious leaves can also affect the life cycle of animals that feed on them.


Ozone is not an equal opportunity pollutant—some plants are highly susceptible to the toxic gas and others less so. In the US, for example, black cherry, quaking aspen, and white pine are among the species most affected. These disparate impacts are behind one of the major impacts of ozone on ecosystems—it changes the composition of the plant community and reduces species richness. Some species of plants may decline or disappear, while others thrive because they no longer have the same competition. Insect and wildlife species that depend on those plants are also affected.


A general view shows the urban area of the Kathmandu Valley amid smoggy conditions in Kathmandu.


Studies in California’s San Bernadino Mountains have shown that ozone increased forest susceptibility to wildfire because ozone-sensitive, fire-resistant species of pine were replaced by species that were more likely to burn.


Chemical aspects of an ecosystem also go through myriad changes as a result of ozone. The gas reduces the amount of nitrogen in the leaves of plants, for example, a key nutrient that drives insect dynamics.

Ozone alters the biogenic volatile organic compounds that are emitted by plants, everything from isoprene to terpenes. These chemicals are how plants signal to other plants, insects, and animals. Relations between insects and the chemicals these plants emit are, the paper says, “highly complex” and are critical to ecosystem functions.

One paper, for example, found that ozone reduced the floral scent that attracts pollinators. These messenger odors tell a bee or other pollinator a half-mile away how much pollen is available, the quality, and what species it is. But higher ozone levels degraded the scent, and pollinators were less successful in finding the target plant. A recent paper found that ozone also diminished the olfactory abilities of pollinators, reducing their ability to detect pollen sources.

Experts say other insects, mammals, and birds are likely to suffer deleterious impacts from ozone, just as humans do. Agathokleous said that research is important not only to understand the threats to existing biodiversity but to guide restoration efforts. But the invisible nature of the threat has been an impediment to attracting the necessary funding.

“Climate change can be seen or felt, whether it’s increased rain, drought, or heat,” says Agathakleous. “Ozone pollution is a hidden problem. It cannot commonly be seen or felt. People don’t pay attention to something they cannot see.”


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